Numerous optical devices utilize elements whose index of refraction is altered in response to a control signal. For example switching elements based on total internal reflection (TIR) switching elements are well known in the optical arts. A TIR element consists of a waveguide with a switchable boundary. Light strikes the boundary at an angle. In the first state, the boundary separates two regions having substantially different indices of refraction. In this state the light is reflected off of the boundary and thus changes direction. In the second state, the two regions separated by the boundary have the same index of refraction and the light continues in a straight line through the boundary. The magnitude of the change of direction depends on the difference in the index of refraction of the two regions. To obtain a large change in direction the region behind the boundary must be switchable between an index of refraction equal to that of the waveguide and an index of refraction that differs markedly from that of the waveguide.
Prior art TIR elements that provide a large change in index of refraction independent of the polarization of the light signal operate by mechanically changing the material behind the boundary, and hence, have relatively slow switching speeds. TIRs based on liquid crystals have higher switching speeds and provide relatively large changes in the index of refraction. However, these devices require the light signal to be polarized, and hence, half of the light intensity must be sacrificed or two devices must be utilized, one operating on each polarization state.
Prior art TIR elements with very fast switching times are also known. These elements alter the index of refraction of the material behind the boundary by applying an electric field to a material whose index of refraction is a function of the electric field. For example, U.S. Pat. No. 5,078,478 describes a TIR element in which the waveguide is constructed in a ferroelectric material. The index of refraction of the ferroelectric material along a boundary within the waveguide is altered by applying an electric field across a portion of the waveguide. While this type of device switches in nanoseconds, the change in index of refraction is very small, and hence, the direction of the light can only be altered by a few degrees. Deflections of this magnitude complicate the design of devices based on TIRs such as cross-point arrays, and hence, commercially viable cross-connects based on this technology have not been forthcoming.
In addition to their use in switching elements, devices that have an electrically controllable index of refraction can also be used to provide optical delay lines and other devices that depend on the transit time of the light. For example, delay lines can be inserted into laser cavities to provide a means for tuning the output wavelength of the laser. Tunable optical filters may also be constructed from such devices. Unfortunately, the problems discussed above with respect to TIRs also inhibit the realization of these applications using prior art optical elements whose index of refraction may be controlled electrically.
Broadly, it is the object of the present invention to provide an improved optical element whose index of refraction can be tuned electrically.
It is a further object of the present invention to provide an optical element having an index of refraction that can be tuned over a larger range than prior art devices without requiring the light signals processed thereby to be polarized.
It is a still further object of the present invention to provide an optical element whose index of refraction can be altered in a shorter time than prior art devices based on mechanical alteration of a portion of a waveguide while providing larger changes of index of refraction than obtainable from ferroelectric and like materials.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.